Just a house that happens to be long in shape. There are specific styles of 'longhouse' in various parts of the world, though - look at the information about them on the web - that would probably also be described as tŷ hir in Welsh.
The long house is very much a feature of Welsh architecture. The idea was that the human occupants had one end of the long house and the cattle had the other end behind a partition. There are surviving examples, but best seen at St Fagans:
It's a fascinating subject and one of my ambitions is to be able to understand the excellent S4C programme Y Ty Cymreig without the subtitles!
Tŷ Mawr in Penmachno seems like it may also be an example of this type of house. Birth place of William Morgan